Certain kinds of electromagnetic radiation detectors, for example semiconductor photodiodes, are known which have connections on both sides of the substrate, and have resistive or capacitive structures in between those connections. The high series resistance between contacts on the side of the substrate may adversely affect the noise performance of these devices. Any antireflective coating may add to this series resistance.
Back illuminated photodiode structures are described in U.S. Pat. No. 6,025,585. U.S. Pat. Nos. 6,670,258 and 6,736,416 show backside illuminated photodiodes that include backside bias electrode layers.
In such backside illuminated photodiodes, the light enters the device through the rear, so that the bias electrode layer and/or the back contact layer must be optically transparent to act as an optical window. The term back contact window is a generic term for a doped backside layer that acts as an optical window.
U.S. Pat. No. 6,504,178 describes an indirect back surface contact the provides an electrical connection to the flat side window of a backside illuminated photodiode. The indirect back surface contact is through undepleted substrate materials outside the depletion region's of the individual photodiodes. The connection may also be made through guard structures of those photodiodes. This type of contact permits the use of insulating antireflective coating structures to improve quantum efficiency.
A back contact indirect structure may improve the quantum efficiency as noted above, and may also simplify attachment of the device to a printed circuit board, since all electrical contacts are made on one side of the substrate.